DeafMetal

I think you'd be better off making a GTA clone on Android or iOS. GBA is kind of older technology now, it was 2 generations ago as far as portable systems are concerned. If you make one for the iOS/Android, you could stand to make significant amounts of money. You would also have more tools available to you and tutorials.

Game like GTA Advance is actually a lot of work to write. Especially if you want it to be even better. You could probably get sufficiently competent programmers in on it if there is opportunity to make profit, but not if it's going to be work for free type of thing.

DeafMetal

It's making an entire game on old hardware with little support, man, and no incentive for anyone to help. It's literally months, even years of work for a team of programmers. I honestly, again, say that if you really want to do this, do it on ios/android. Give it out for free with optional micro-transactions, split the profit among the team. What you're wanting is pretty far-fetched unless you wanna work on it yourself for several years, and have the end-product for yourself and VERY few others to enjoy.

From what you're saying, I see absolutely no reason why anyone would want to help. You are not offering monetary incentive, you're working on an unnecessarily complicated platform that offers comparably little support for new-comers, and you're not even offering them the potential of recognition, as the GBA is pretty damn old, and few people will end up playing your game. It's like making an entire clone of GTA London for the PS1 at this day and age.